Rainfall deciles for April-September. From http://www.bom.gov.au/climate/maps/

If you live in NSW, you might have noticed it’s been pretty dry lately. In Sydney we’ve had our driest September on record, and the last time we had real rain was two whole months ago, the longest spell on record. After a very wet March, April-September has been the driest for Sydney in over a decade.

In the absence of El Niño, we look to other explanations, like the very high pressure in June. And for the east coast, it feels like we just haven’t had any ECLs. But is that true?

Conveniently, NCEP in the USA provides grids of “reanalysis” pressure data almost in real time, so I ran my tracking scheme and had a look! During April-September, which is the main ECL season (as well as the period it’s been so dry), typically there are 28 days with a low pressure system somewhere in the ECL region. This year there were 24 – not that much different to normal.

All cyclones April-September 2017. Crosses mark strong systems

I know what you’re thinking – you would remember if that had happened! As it turns out, our standard ECL box extends pretty far east, and this year the systems were generally too far east (or south) to affect the coast.

Only 12 days this year had an system closer to the coast, the fewest since 2003. And only five were north of 38°S, the main one being a pretty weak low on the coast that contributed to rain during early June.

In fact, they were all pretty weak. Only four days had an ECL reach a strong threshold anywhere in the domain, and just one in the closer region (and it was very far south).

Usually there are 12 days with a strong ECL; last year there were 18 days, including 15 with a strong ECL near the coast and a pretty severe one in June! Amusingly, this means that while this year had the fewest strong ECLs since 1994, last year had the most since 1990, really showcasing how variable ECLs are!

Number of April-September ECLs from the NCEP reanalysis.

But why?

Well, I'd like to tell you. Unfortunately, we actually don't have a very good understanding of why and how ECLs vary from year to year. There's some evidence that they're more likely in La Niña years, especially the ones with very heavy rain and flooding - but generally their relationship with the big climate drivers is pretty weak, with some very significant ECLs during big El Niño years like 2015.

While there is some relationship with changes in pressure patterns such as SAM, this year hasn't been that much different to the last couple, which had heaps of ECLs. And that really high pressure from June didn't really continue into the rest of winter.

So I actually have no idea. It could just be random chance, or maybe there are climate patterns we just don't understand yet. But it would be an interesting thing to know!